The power factor (PF) of an alternating current (AC) electric circuit is the ratio of real power to apparent power, and is expressed as a number between 0 and 1.0 (or as a percentage). Real power is the capacity of the circuit to perform work in a given time, and apparent power is the product of the current and voltage of the circuit. Various factors (e.g., a non-linear load, or the amount of energy stored in the load versus energy returned to the power source) can cause the apparent power to exceed the real power, increasing power losses through the utility company's electrical transmission and distribution lines. Utility companies may even charge higher rates to customers who do not maintain high power factors.
Accordingly, it is often desirable to adjust the power factor of an electronics system (e.g., a computer server or collection of servers such as a “server farm”). Power factor correction (PFC) circuits are available that bring the power factor of an AC circuit closer to 1.0. Typical PFC circuits operate by determining the PF and adjusting input current so that the current is in phase with the supply voltage. The PF can be determined by dividing the power (in Watts) by the product of measured voltage and current. Therefore, it is important for the voltage and current measurements to be accurate.
Sensing elements for measuring voltage and current in PFC circuits are generally sized to minimize power loss for the highest rated output power at the lowest rated input voltage. But when operating at the lowest rated output power at the highest rated input voltage, the voltage signal generated by the current sensing element can be very small, with a poor signal to noise ratio. The decreased accuracy in this measurement makes it more difficult, if not impossible, to maintain an optimal PF for variable loads (e.g., based on system design, operating conditions, etc.).
PFC circuits may be turned on and off randomly as needed to support the load, thereby saving energy. When randomly turning on and off the PFC circuit, however, PFC suffers and it is difficult if not impossible to accurately measure the current. It is also possible to increase the sense resistance, but this typically increases power loss and reduces operating efficiency. In addition, PFC and current measurement may improve, but again degrades at lower current levels.